Carbon nanospikes have improved sensitivity and antifouling properties for adenosine, hydrogen peroxide, and histamine.

Carbon nanospikes (CNSs) are a new nanomaterial that has enhanced surface roughness and surface oxide concentration, increasing the sensitivity for dopamine detection. However, CNS-modified electrodes (CNSMEs) have not been characterized for other neurochemicals, particularly those with higher oxidation potentials. The purpose of this study was to evaluate CNSMEs for the detection of adenosine, hydrogen peroxide (H2O2), and histamine. The sensitivity increased with CNSs, and signals at CNSMEs were about 3.3 times higher than CFMEs. Normalizing for surface area differences using background currents, CNSMEs show an increased signal of 4.8 times for adenosine, 1.5 times for H2O2, and 2 times for histamine. CNSMEs promoted the formation of secondary products for adenosine and histamine, which enables differentiation from other analytes with similar oxidation potentials. CNSs also selectively enhance the sensitivity for adenosine and histamine compared to H2O2. A scan rate test reveals that adenosine is more adsorption-controlled at CNS electrodes than CFMEs. CNSMEs are antifouling for histamine, with less fouling because the polymers formed after histamine electrooxidation do not adsorb due to an elevated number of edge planes. CNSMEs were useful for detecting each analyte applied in brain slices. Because of the hydrophilic surface compared to CFMEs, CNSMEs also have reduced biofouling when used in tissue. Therefore, CNSMEs are useful for tissue measurements of adenosine, hydrogen peroxide, and histamine with high selectivity and low fouling.

Carbon nanospike coated nanoelectrodes for measurements of neurotransmitters.

Carbon nanoelectrodes enable the detection of neurotransmitters at the level of single cells, vesicles, synapses and small brain structures. Previously, the etching of carbon fibers and 3D printing based on direct laser writing have been used to fabricate carbon nanoelectrodes, but these methods lack the ability of mass manufacturing. In this paper, we mass fabricate carbon nanoelectrodes by growing carbon nanospikes (CNSs) on metal wires. CNSs have a short, dense and defect-rich surface that produces remarkable electrochemical properties, and they can be mass fabricated on almost any substrate without using catalysts. Tungsten wires and niobium wires were electrochemically etched in batch to form sub micrometer sized tips, and a layer of CNSs was grown on the metal wires using plasma-enhanced chemical vapor deposition (PE-CVD). The thickness of the CNS layer was controlled by the deposition time, and a thin layer of CNSs can effectively cover the entire metal surface while maintaining the tip size within the sub micrometer scale. The etched tungsten wires produced tapered conical nanotips, while the etched niobium wires were long and thin. Both showed excellent sensitivity for the detection of outer sphere ruthenium hexamine and the inner sphere test compound ferricyanide. The CNS nanosensors were used for the measurement of dopamine, serotonin, ascorbic acid and DOPAC with fast-scan cyclic voltammetry. The CNS nanoelectrodes had a large surface area and numerous defect sites, which improved the sensitivity, electron transfer kinetics and adsorption. Finally, the CNS nanoelectrodes were compared with other nanoelectrode fabrication methods, including flame etching, 3D printing, and nanopipettes, which are slower to make and more difficult for mass fabrication. Thus, CNS nanoelectrodes are a promising strategy for the mass fabrication of nanoelectrode sensors for neurotransmitters.

Influence of Geometry on Thin Layer and Diffusion Processes at Carbon Electrodes.

The geometric structure of carbon electrodes affects their electrochemical behavior, and large-scale surface roughness leads to thin layer electrochemistry when analyte is trapped in pores. However, the current response is always a mixture of both thin layer and diffusion processes. Here, we systematically explore the effects of thin layer electrochemistry and diffusion at carbon fiber (CF), carbon nanospike (CNS), and carbon nanotube yarn (CNTY) electrodes. The cyclic voltammetry (CV) response to the surface-insensitive redox couple Ru(NH3)63+/2+ is tested, so the geometric structure is the only factor. At CFs, the reaction is diffusion-controlled because the surface is smooth. CNTY electrodes have gaps between nanotubes that are about 10 µm deep, comparable with the diffusion layer thickness. CNTY electrodes show clear thin layer behavior due to trapping effects, with more symmetrical peaks and ΔEp closer to zero. CNS electrodes have submicrometer scale roughness, so their CV shape is mostly due to diffusion, not thin layer effects. However, even the 10% contribution of thin layer behavior reduces the peak separation by 30 mV, indicating ΔEp is influenced not only by electron transfer kinetics but also by surface geometry. A new simulation model is developed to quantitate the thin layer and diffusion contributions that explains the CV shape and peak separation for CNS and CNTY electrodes, providing insight on the impact of scan rate and surface structure size. Thus, this study provides key understanding of thin layer and diffusion processes at different surface structures and will enable rational design of electrodes with thin layer electrochemistry.

Solvent-Free Synthesis of CuO/HKUST-1 Composite and Its Photocatalytic Application.

A metal-organic framework (MOF) is one kind of crystalline microporous material and is increasingly used as a host of catalytically active guests. Nanostructured materials supported on MOFs have presented enhanced catalytic activity and stability. Templates or several steps are essential to the synthesis of MOF composites. Simple and effective MOF synthesis methods are still challenging. Nanosized copper oxide particles in MOF composites, described as nanosized CuO@HKUST-1, were prepared by a facile solvent-free reaction. These series of CuO@HKUST-1 composites exhibited excellent photocatalytic activity for production of hydrogen and methylene blue (MB) degradation under visible light. This synthesis method provides an effective way to fabricate MOF-related nanocomposite catalysts.

Carbon nanospikes have better electrochemical properties than carbon nanotubes due to greater surface roughness and defect sites.

Carbon nanomaterials are used to improve electrodes for neurotransmitter detection, but what properties are important for maximizing those effects? In this work, we compare a newer form of graphene, carbon nanospikes (CNSs), with carbon nanotubes (CNTs) grown on wires and carbon fibers (CFs). CNS electrodes have a short, dense, defect-filled surface that produces remarkable electrochemical properties, much better than CNTs or CFs. The CNS surface roughness is 5.5 times greater than glassy carbon, while CNTs enhance roughness only 1.8-fold. D/G ratios are higher for CNS electrodes than CNT electrodes, an indication of more defect sites. For cyclic voltammetry of dopamine and ferricyanide, CNSs have both higher currents and smaller ΔEp values than CNTs and CFs. CNS electrodes also have a very low resistance to charge transfer. With fast-scan cyclic voltammetry (FSCV), CNS electrodes have enhanced current density for dopamine and cationic neurotransmitters due to increased adsorption to edge plane sites. This study establishes that not all carbon nanomaterials are equally advantageous for dopamine electrochemistry, but that short, dense nanomaterials that add defect sites provide improved current and electron transfer. CNSs are simple to mass fabricate on a variety of substrates and thus could be a favorable material for neurotransmitter sensing.

Pressure-Induced Diels-Alder Reactions in C6 H6 -C6 F6 Cocrystal towards Graphane Structure.

Pressure-induced polymerization (PIP) of aromatics is a novel method for constructing sp3 -carbon frameworks, and nanothreads with diamond-like structures were synthesized by compressing benzene and its derivatives. Here by compressing a benzene-hexafluorobenzene cocrystal (CHCF), H-F-substituted graphane with a layered structure in the PIP product was identified. Based on the crystal structure determined from the in situ neutron diffraction and the intermediate products identified by gas chromatography-mass spectrum, we found that at 20 GPa CHCF forms tilted columns with benzene and hexafluorobenzene stacked alternatively, and leads to a [4+2] polymer, which then transforms to short-range ordered H-F-substituted graphane. The reaction process involves [4+2] Diels-Alder, retro-Diels-Alder, and 1-1' coupling reactions, and the former is the key reaction in the PIP. These studies confirm the elemental reactions of PIP of CHCF for the first time, and provide insight into the PIP of aromatics.

Polymeric spatial resolution test patterns for mass spectrometry imaging using nano-thermal analysis with atomic force microscopy.

RATIONALE: As the spatial resolution of mass spectrometry imaging technologies has begun to reach into the nanometer regime, finding readily available or easily made resolution reference materials has become particularly challenging for molecular imaging purposes. This paper describes the fabrication, characterization and use of vertical line array polymeric spatial resolution test patterns for nano-thermal analysis/atomic force microscopy/mass spectrometry chemical imaging. METHODS: Test patterns of varied line width (0.7 or 1.0 µm) and spacing (0.7 or 1.0 µm) were created in an ~1-µm-thick poly(methyl methacrylate) thin film using electron beam lithography. The patterns were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy topography and nano-thermal analysis/mass spectrometry imaging. RESULTS: The efficacy of these polymeric test patterns for the advancement of chemical imaging techniques was illustrated by their use to judge the spatial resolution improvement achieved by heating the ionization interface of the current instrument platform. The spatial resolution of the mass spectral chemical images was estimated to be 1.4 µm, based on the ability to statistically distinguish 0.7-µm-wide lines separated by 0.7-µm-wide spacings in those images when the interface cross was heated to 200°C. CONCLUSIONS: This work illustrates that e-beam lithography is a viable method to create spatial resolution test patterns in a thin film of high molecular weight polymer to allow unbiased judgment of intra-laboratory advancement and/or inter-laboratory comparison of instrument advances in nano-thermal analysis/atomic force microscopy/mass spectrometry chemical imaging. Published in 2017. This article is a U.S. Government work and is in the public domain in the USA.

Superior Conductive Solid-like Electrolytes: Nanoconfining Liquids within the Hollow Structures.

The growth and proliferation of lithium (Li) dendrites during cell recharge are currently unavoidable, which seriously hinders the development and application of rechargeable Li metal batteries. Solid electrolytes with robust mechanical modulus are regarded as a promising approach to overcome the dendrite problems. However, their room-temperature ionic conductivities are usually too low to reach the level required for normal battery operation. Here, a class of novel solid electrolytes with liquid-like room-temperature ionic conductivities (>1 mS cm(-1)) has been successfully synthesized by taking advantage of the unique nanoarchitectures of hollow silica (HS) spheres to confine liquid electrolytes in hollow space to afford high conductivities (2.5 mS cm(-1)). In a symmetric lithium/lithium cell, the solid-like electrolytes demonstrate a robust performance against the Li dendrite problem, preventing the cell from short circuiting at current densities ranging from 0.16 to 0.32 mA cm(-2) over an extended period of time. Moreover, the high flexibility and compatibility of HS nanoarchitectures, in principle, enables broad tunability to choose desired liquids for the fabrication of other kinds of solid-like electrolytes, such as those containing Na(+), Mg(2+), or Al(3+) as conductive media, providing a useful alternative strategy for the development of next generation rechargeable batteries.

Optical and infrared properties of glancing angle-deposited nanostructured tungsten films.

Nanotextured tungsten thin films were obtained on a stainless steel (SS) substrate using the glancing-angle-deposition (GLAD) method. It was found that the optical absorption and thermal emittance of the SS substrate can be controlled by varying the parameters used during deposition. Finite-difference time-domain (FDTD) simulations were used to predict the optical absorption and infrared (IR) reflectance spectra of the fabricated samples, and good agreement was found between simulated and measured data. FDTD simulations were also used to predict the effect of changes in the height and periodicity of the nanotextures. These simulations show that good control over the absorption can be achieved by altering the height and periodicity of the nanostructure. These nanostructures were shown to be temperature stable up to 500°C with the addition of a protective HfO2 layer. Applications for this structure are explored, including a promising application for solar thermal energy systems.

Carbon nanospikes grown on metal wires as microelectrode sensors for dopamine.

Carbon nanomaterials are advantageous as electrodes for neurotransmitter detection, but the difficulty of nanomaterials deposition on electrode substrates limits the reproducibility and future applications. In this study, we used plasma enhanced chemical vapor deposition (PECVD) to directly grow a thin layer of carbon nanospikes (CNS) on cylindrical metal substrates. No catalyst is required and the CNS surface coverage is uniform over the cylindrical metal substrate. The CNS growth was characterized on several metallic substrates including tantalum, niobium, palladium, and nickel wires. Using fast-scan cyclic voltammetry (FSCV), bare metal wires could not detect 1 µM dopamine while carbon nanospike coated wires could. The highest sensitivity and optimized S/N ratio was recorded from carbon nanospike-tantalum (CNS-Ta) microwires grown for 7.5 minutes, which had a LOD of 8 ± 2 nM for dopamine with FSCV. CNS-Ta microelectrodes were more reversible and had a smaller ΔE(p) for dopamine than carbon-fiber microelectrodes, suggesting faster electron transfer kinetics. The kinetics of dopamine redox were adsorption controlled at CNS-Ta microelectrodes and repeated electrochemical measurements displayed stability for up to ten hours in vitro and over a ten day period as well. The oxidation potential was significantly different for ascorbic acid and uric acid compared to dopamine. Growing carbon nanospikes on metal wires is a promising method to produce uniformly-coated, carbon nanostructured cylindrical microelectrodes for sensitive dopamine detection.

Studies on supercapacitor electrode material from activated lignin-derived mesoporous carbon.

We synthesized mesoporous carbon from pre-cross-linked lignin gel impregnated with a surfactant as the pore-forming agent and then activated the carbon through physical and chemical methods to obtain activated mesoporous carbon. The activated mesoporous carbons exhibited 1.5- to 6-fold increases in porosity with a maximum Brunauer-Emmett-Teller (BET) specific surface area of 1148 m(2)/g and a pore volume of 1.0 cm(3)/g. Both physical and chemical activation enhanced the mesoporosity along with significant microporosity. Plots of cyclic voltammetric data with the capacitor electrode made from these carbons showed an almost rectangular curve depicting the behavior of ideal double-layer capacitance. Although the pristine mesoporous carbon exhibited a range of surface-area-based capacitance similar to that of other known carbon-based supercapacitors, activation decreased the surface-area-based specific capacitance and enhanced the gravimetric specific capacitance of the mesoporous carbons. A vertical tail in the lower-frequency domain of the Nyquist plot provided additional evidence of good supercapacitor behavior for the activated mesoporous carbons. We have modeled the equivalent circuit of the Nyquist plot with the help of two constant phase elements (CPE). Our work demonstrated that biomass-derived mesoporous carbon materials continue to show potential for use in specific electrochemical applications.

Sulfur functionalized biocarbon sorbents for low-concentration mercury isolation.

Sulfur functionalized biocarbons were prepared from naturally abundant lignin alkali with sodium thiocyanate as an activation agent and a sulfur source. The resultant biocarbon sorbents showed a high mercury isolation ability from aqueous solutions, where high surface area and doping of sulfur significantly aid the uptake of mercury, i.e., 0.05 g of biocarbon sorbent removed 99% of mercury from 250 mL of simulated wastewater with an initial concentration of mercury of 10 mg L-1.

Top-Down Fabrication of Atomic Patterns in Twisted Bilayer Graphene.

Atomic-scale engineering typically involves bottom-up approaches, leveraging parameters such as temperature, partial pressures, and chemical affinity to promote spontaneous arrangement of atoms. These parameters are applied globally, resulting in atomic-scale features scattered probabilistically throughout the material. In a top-down approach, different regions of the material are exposed to different parameters, resulting in structural changes varying on the scale of the resolution. In this work, the application of global and local parameters is combined in an aberration-corrected scanning transmission electron microscope (STEM) to demonstrate atomic-scale precision patterning of atoms in twisted bilayer graphene. The focused electron beam is used to define attachment points for foreign atoms through the controlled ejection of carbon atoms from the graphene lattice. The sample environment is staged with nearby source materials such that the sample temperature can induce migration of the source atoms across the sample surface. Under these conditions, the electron-beam (top-down) enables carbon atoms in the graphene to be replaced spontaneously by diffusing adatoms (bottom-up). Using image-based feedback control, arbitrary patterns of atoms and atom clusters are attached to the twisted bilayer graphene with limited human interaction. The role of substrate temperature on adatom and vacancy diffusion is explored by first-principles simulations.

Using Vertically Aligned Carbon Nanofiber Arrays on Rigid or Flexible Substrates for Delivery of Biomolecules and Dyes to Plants.

The delivery of biomolecules and impermeable dyes to intact plants is a major challenge. Nanomaterials are up-and-coming tools for the delivery of DNA to plants. As exciting as these new tools are, they have yet to be widely applied. Nanomaterials fabricated on rigid substrate (backing) are particularly difficult to successfully apply to curved plant structures. This study describes the process for microfabricating vertically aligned carbon nanofiber arrays and transferring them from a rigid to a flexible substrate. We detail and demonstrate how these fibers (on either rigid or flexible substrates) can be used for transient transformation or dye (e.g., fluorescein) delivery to plants. We show how VACNFs can be transferred from rigid silicon substrate to a flexible SU-8 epoxy substrate to form flexible VACNF arrays. To overcome the hydrophobic nature of SU-8, fibers in the flexible film were coated with a thin silicon oxide layer (2-3 nm). To use these fibers for delivery to curved plant organs, we deposit a 1 µL droplet of dye or DNA solution on the fiber side of VACNF films, wait 10 min, place the films on the plant organ and employ a swab with a rolling motion to drive fibers into plant cells. With this method, we have achieved dye and DNA delivery in plant organs with curved surfaces.

An efficient and broadly applicable method for transient transformation of plants using vertically aligned carbon nanofiber arrays.

Transient transformation in plants is a useful process for evaluating gene function. However, there is a scarcity of minimally perturbing methods for gene delivery that can be used on multiple organs, plant species, and non-excised tissues. We pioneered and demonstrated the use of vertically aligned carbon nanofiber (VACNF) arrays to efficiently perform transient transformation of different tissues with DNA constructs in multiple plant species. The VACNFs permeabilize plant tissue transiently to allow molecules into cells without causing a detectable stress response. We successfully delivered DNA into leaves, roots and fruit of five plant species (Arabidopsis, poplar, lettuce, Nicotiana benthamiana, and tomato) and confirmed accumulation of the encoded fluorescent proteins by confocal microscopy. Using this system, it is possible to transiently transform plant cells with both small and large plasmids. The method is successful for species recalcitrant to Agrobacterium-mediated transformation. VACNFs provide simple, reliable means of DNA delivery into a variety of plant organs and species.

Effective antiviral coatings for deactivating SARS-CoV-2 virus on N95 respirator masks or filters.

The application of antiviral coatings to masks and respirators is a potential mitigating step toward reducing viral transmission during the SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) pandemic. The use of appropriate masks, social distancing, and vaccines is the immediate solution for limiting the viral spread and protecting people from this virus. N95 respirator masks are effective in filtering the virus particles, but they cannot kill or deactivate the virus. We report a possible approach to deactivating SARS-CoV-2 by applying an antimicrobial coating (Goldshield 75) to masks and respirators, rendering them suitable for repeated use. Masks coated with Goldshield 75 demonstrated continuous inactivation of the Alpha and Beta variants of the SARS-CoV-2 over a 3-day period and no loss of inactivation when stored at temperatures at 50 °C.

Electrical and thermal conductivity of low temperature CVD graphene: the effect of disorder.

In this paper we present a study of graphene produced by chemical vapor deposition (CVD) under different conditions with the main emphasis on correlating the thermal and electrical properties with the degree of disorder. Graphene grown by CVD on Cu and Ni catalysts demonstrates the increasing extent of disorder at low deposition temperatures as revealed by the Raman peak ratio, IG/ID. We relate this ratio to the characteristic domain size, La, and investigate the electrical and thermal conductivity of graphene as a function of La. The electrical resistivity, ρ, measured on graphene samples transferred onto SiO2/Si substrates shows linear correlation with La(-1). The thermal conductivity, K, measured on the same graphene samples suspended on silicon pillars, on the other hand, appears to have a much weaker dependence on La, close to K∼La1/3. It results in an apparent ρ∼K3 correlation between them. Despite the progressively increasing structural disorder in graphene grown at lower temperatures, it shows remarkably high thermal conductivity (10(2)-10(3) W K(-1) m(-1)) and low electrical (10(3)-3×10(5) Ω) resistivities suitable for various applications.

Carbonized Polymer for Joule Heating Processing Towards Biosensor Development.

This paper presents the experimental findings towards developing carbonized microelectrodes using a Joule heating process within a temperature window that is compatible with CMOS. Bridge-on-pillars polymer structures have been 3D-printed using two-photon polymerization (2PP). They have been annealed in various processing conditions to increase the fraction of carbon in the precursor material and to achieve appreciable electric conductivity so that they can be used to drive current to enable Joule heating. To evaluate the outcome of the processing sequences, Raman spectroscopy has been performed to assess the degree of carbonization. Such CMOS-compatible carbon electrodes are important for monolithic, low-cost biosensor development.Clinical relevance- This establishes the potential of carbonized polymer electrode for low-cost, CMOS-compatible monolithic biosensor platform for implementation in medical diagnosis and treatment.

Performing In Situ Closed-Cell Gas Reactions in the Transmission Electron Microscope.

Gas reactions studied by in situ electron microscopy can be used to capture the real-time morphological and microchemical transformations of materials at length scales down to the atomic level. In situ closed-cell gas reaction (CCGR) studies performed using (scanning) transmission electron microscopy (STEM) can separate and identify localized dynamic reactions, which are extremely challenging to capture using other characterization techniques. For these experiments, we used a CCGR holder that utilizes microelectromechanical systems (MEMS)-based heating microchips (hereafter referred to as "E-chips"). The experimental protocol described here details the method for performing in situ gas reactions in dry and wet gases in an aberration-corrected STEM. This method finds relevance in many different materials systems, such as catalysis and high-temperature oxidation of structural materials at atmospheric pressure and in the presence of various gases with or without water vapor. Here, several sample preparation methods are described for various material form factors. During the reaction, mass spectra obtained with a residual gas analyzer (RGA) system with and without water vapor further validates gas exposure conditions during reactions. Integrating an RGA with an in situ CCGR-STEM system can, therefore, provide critical insight to correlate gas composition with the dynamic surface evolution of materials during reactions. In situ/operando studies using this approach allow for detailed investigation of the fundamental reaction mechanisms and kinetics that occur at specific environmental conditions (time, temperature, gas, pressure), in real-time, and at high spatial resolution.

Polymer, Additives, and Processing Effects on N95 Filter Performance.

The current severe acute respiratory syndrome coronavirus 2 (SARS-COV-2) pandemic has highlighted the need for personal protective equipment, specifically filtering facepiece respirators like N95 masks. While it is common knowledge that polypropylene (PP) is the industry standard material for filtration media, trial and error is often required to identify suitable commercial precursors for filtration media production. This work aims to identify differences between several commercial grades of PP and demonstrate the development of N95 filtration media with the intent that the industry partners can pivot and help address N95 shortages. Three commercial grades of high melt flow index PP were melt blown at Oak Ridge National Laboratory and broadly characterized by several methods including differential scanning calorimetry (DSC), X-ray diffraction (XRD), and neutron scattering. Despite the apparent similarities (high melt flow and isotacticity) between PP feedstocks, the application of corona charging and charge enhancing additives improve each material to widely varying degrees. From the analysis performed here, the most differentiating factor appears to be related to crystallization of the polymer and the resulting electret formation. Materials with higher crystallization onset temperatures, slower crystallization rates, and larger number of crystallites form a stronger electret and are more effective at filtration.